The cellular biology underlying fracture repair is quite remarkable. Despite high energy injuries, wide displacement at the fracture site, dramatic comminution of the bone, and even segmental bone loss, most fracture heal in an uncomplicated fashion and patients frequently recover most, if not all, of their pre-injury functio. Yet despite the skeleton's robust healing response, some 5 to 8% of patients do not heal their fractures, progressing on to nonunion. From a clinical standpoint, this group of patients with nonunion suffer equally dramatic disability, frequently requiring multiple surgeries, with increased morbidity and even loss of limb. For severe injuries, such as open tibia fractures, reoperation rates exceed 60% in some series, and patient reported outcomes even after the fracture is healed remain uniformly poor. It is widely recognized from both a basic science and a clinical perspective that smoking tobacco adversely affects fracture healing. While this association between poor healing and smoking is clear, the underlying cause of the toxicity in tobacco smoke is not known. Recent work has highlighted the role of hypoxic signaling through the Hypoxia Inducible Factor (HIF) pathway as critical for chondrocyte development and fracture repair, leading to the hypothesis that the impaired fracture healing in smokers is related to carbon monoxide exposure and disordered HIF signaling. We further hypothesize that this effect can be ameliorated by the use of prolyl hydroxylase inhibitors, restoring the cellular response to hypoxia and allowing for the accumulation of HIF products. As a practicing orthopedic trauma surgeon in an academic practice, I have spent several years developing the experience and skills necessary to pursue my interests investigating the cellular physiology underlying fracture repair. I have been mentored in this pursuit, and have conducted in vivo fracture studies using an established animal model. Through this proposal, I seek to further develop the basic science training and skill set I will need to pursue high quality and clinically relevant investigations asan independent investigator. In addition, identifying causative toxins in tobacco smoke has clear clinical implications, potentially supporting the use of transdermal nicotine and/or pharmacologic treatment to overcome the complications seen in this high risk group of patients. PUBLIC HEALTH RELEVANCE: Smoking is associated with impaired fracture healing, yet the causative toxins in tobacco smoke have not been identified. This proposal seeks to test the hypothesis that smoking cigarettes impairs the ability of cells to sense low oxygen states, which is required for induction of endochondral bone repair. Thus, in the presence of carbon monoxide, mesenchymal stem cells may fail to respond to hypoxic conditions which under normal circumstances would drive terminal differentiation to hypertrophic chondrocytes. Restoring the cellular response to hypoxia could have important consequences for those fracture patients at high risk for nonunion and poor outcome after injury.